1. Field of the Invention
The present invention relates to a method and apparatus for recording and/or reading image information by using a photoconductor which generates electric charges when exposed to an electromagnetic wave such as a radiation and light.
2. Description of the Related Art
Conventionally, a number of systems are proposed for recording and/or reading image information by employing a photoconductor realized by an organic or inorganic amorphous semiconductor material, and utilizing the property of exhibiting electric conductivity (i.e., generating pairs of electric charges in the photoconductor) when the photoconductor is exposed to electromagnetic waves. For example, the above systems are disclosed in the coassigned U.S. Ser. No. 09/792,035 corresponding to Japanese Patent Application No. 2000-50201, the coassigned U.S. Ser. No. 09/534,204 corresponding to Japanese Patent Application Nos. 2000-50202, 2000-50203, 2000-50204, 2000-50205, and 11(1999)-79984, the coassigned U.S. Ser. No. 09/136,739, now U.S. Pat. No. 6,268,614, corresponding to Japanese Unexamined Patent Publication No. 2000-105297, the coassigned U.S. Ser. No. 09/539,412 corresponding to Japanese Unexamined Patent Publication No. 2000-284056, the coassigned U.S. Ser. No. 09/538,479 corresponding Japanese Unexamined Patent Publication No. 2000-284057, the U.S. Pat. Nos. 5,648,660, 5,661,309, and 4,535,468, Japanese Unexamined Patent Publication No. 9(1997)-206293, and Medical Physics, Vol. 16, No.1, January/February 1989, pp.105-109.
The recording systems for recording image information have a laminated structure in which a photoconductor is sandwiched between two electrodes, and a charge storing portion is provided for storing charges generated in the photoconductor. When the photoconductor is exposed to electromagnetic waves (or recording light) carrying image information while applying a voltage between the two electrodes so as to produce an electric field in the photoconductor, pairs of charges are generated in the photoconductor, and latent-image charges in the generated pairs are stored in the charge storing portion. Thus, the image information is recorded as a latent image.
On the other hand, in the reading systems for reading information, charges are generated in a photoconductor when the photoconductor is exposed to electromagnetic waves carrying image information while applying an electric field to the photoconductor, and the image information is read by detecting the generated charges, i.e., detecting currents produced by the generated charges. The electromagnetic wave are, for example, X rays which have penetrated through an object, or accelerated phosphorescence light emitted from a stimulable phosphor sheet used as an image recording medium.
However, in the case where an amorphous material such as a-Se (amorphous selenium) is used in each of the above photoconductors, charges are directly injected from the electrodes located on both sides of the photoconductor into the photoconductor from the beginning of application of a voltage (which is generally high) between the electrodes until short-circuiting of the electrodes. A portion of the injected charges is trapped as space charges in the photoconductor or at the interfaces between the photoconductor and the electrodes, and the other portion of the injected charges is not trapped, and output from the photoconductor as a leakage current. Thus, a dark current flows in the photoconductor.
In the recording systems, unnecessary charges caused by the dark current are accumulated in the charge storing portion. Therefore, a dark latent image, which is produced by the unnecessary charges, and does not carry true image information, is superimposed on a true latent image corresponding to the true image information. Thus, when the charges stored in the charge storing portion is read after the recording operation, the dark latent image produced during the recording operation appears as dark latent image noise in a regenerated image.
On the other hand, in the reading systems, the dark current flowing in the photoconductor is superimposed on a true current component carrying the true image information. Therefore, the dark current flowing in the photoconductor during the reading operation also appears as dark latent image noise in a regenerated image.
In particular, since the quantum efficiency of the photoconductor with respect to X rays is low, the amount of charges generated by direct exposure to X rays which have penetrated through an object is very small. In addition, since the accelerated phosphorescence light is very weak, the amount of charges generated by exposure to the accelerated phosphorescence light is also very small. Therefore, in these cases, when the dark current is large, the S/N ratio decreases seriously.
If the dark current can be reduced, the influence of the dark latent image noise is also reduced, and the decrease in the S/N ratio can be prevented. However, in order to reduce the dark current, the dark resistance must be increased. For example, in the case where a detector which includes an a-Se photoconductor having a thickness of 500 micrometers is exposed to a 10 mR dose of radiation having energy of 80 keV for one second, the magnitude of the dark current must be reduced to 10 pA/cm2 or less in order to reduce the influence of the dark latent image to an ignorable degree. In order to achieve such reduction of the dark current, the dark resistance must be increased to a very great value as much as 1015 xcexa9.cm or more when an electric field of 10 V/xcexcm is applied to the photoconductor.
Although a-Se is usable under the dark resistance of 1015 xcexa9.cm in the electric field of 10 V/xcexcm, the dark resistance of 1015 xcexa9.cm is insufficient to achieve a satisfactory S/N ratio in the regenerated image in the case where recording or reading is performed by direct exposure of the photoconductor to X rays or exposure of the photoconductor to accelerated phosphorescence light. Therefore, a higher dark resistance is required. Conventionally, increase of the dark resistance by appropriate selection of a material for the electrodes is proposed, for example, by R. E. Johanson et al. (xe2x80x9cMetallic Electrical Contacts to Stabilized Amorphous Selenium for Use in X-ray Image Detectors,xe2x80x9d Journal of Non-Crystalline Solids, Vol. 227-230 (1998) pp. 1359-1362). In addition, increase of the dark resistance by arrangement of an appropriate blocking layer between the photoconductor and the electrodes (which is made of, for example, a-Se) is proposed, for example, by B. Polischuk et al., (xe2x80x9cSelenium Direct Converter Structure for Static and Dynamic X-ray Detection in Medical Imaging Application,xe2x80x9d Proceedings of the SPIE Conference on Physics of Medical Imaging, February 1998, SPIE Vol. 3336, Paper #: 3336-51).
Conventionally, the strength of the electric field which is most often applied to the photoconductor is 10 V/xcexcm. However, when the strength of the electric field of the photoconductor is increased beyond 10 V/xcexcm in order to increase the quantum efficiency and sensitivity by causing the avalanche amplification, the dark current often increases more than the true signal component, and therefore the SIN ratio decreases.
On the other hand, the dark current has a characteristic that a very large, momentary charging current first flows at the beginning of the application of an electric field. Thereafter, a transient current (absorption current) flows, where the absorption current gradually decreases with time to a constant leakage current. In other words, the dark resistance at the beginning of the application of the electric field is smaller than the dark resistance in a high resistance state, in which the stabilized low leakage current flows. The higher the application voltage is, the more pronounced the above phenomenon is. As disclosed in the R. E. Johanson reference, it takes a relatively long time to reach a stable, high resistance state after voltage application. For example, it takes usually one to ten minutes, and in some instances about one hour. Further, a long start-up time is required to use the photoconductor in the high resistance state.
An object of the present invention is to provide a method and an apparatus for recording image information, which can reduce a dark current flowing during application of an electric field for recording.
Another object of the present invention is to provide a method and an apparatus for reading image information, which can reduce a dark current flowing during application of an electric field for reading.
Still another object of the present invention is to provide a method and an apparatus for recording image information, which can reduce a time required to reach a stable, high resistance state in which the dark current level is low.
A further object of the present invention is to provide a method and an apparatus for reading image information, which can reduce a time required to reach a stable, high resistance state in which the dark current level is low.
(1) According to the first aspect of the present invention, there is provided a method for recording image information in an image recording medium including a photoconductor which is made of an amorphous material, and generates latent-image charges when the photoconductor is exposed to an electromagnetic wave during application of a first electric field to the photoconductor, and a charge storing portion which stores the latent-image charges so as to form a latent image. The method comprises the steps of: (a) applying to the photoconductor a second electric field stronger than the first electric field; (b) stopping application of the second electric field to the photoconductor; and (c) recording in the image recording medium the image information carried by the electromagnetic wave by exposing the photoconductor to the electromagnetic wave while applying the first electric field to the photoconductor, so that the latent-image charges generated in the photoconductor corresponding to the image information are stored in the charge storing portion.
When the application of the second electric is stopped in the step (b), the electric field applied to the photoconductor is reduced to the strength of the first electric field or less. For example, the electric field applied to the photoconductor may be reduced to the strength of the first electric field, or to zero.
Preferably, the method according to the first aspect of the present invention also has one or any possible combination of the following additional features (i) to (iii).
(i) The step (c) may be performed within about thirty seconds of the operation of the step (b).
(ii) The method according to the first aspect of the present invention may further comprise the step of (d) applying a third electric field to the photoconductor before performing the step (a), where the third electric field has an identical strength to the first electric field.
In addition, it is further preferable that the second electric field is applied after a dark current which flows in response to the application of the third electric field becomes stable (i.e., after the photoconductor enters a high resistance state).
(iii) The method according to the first aspect of the present invention may further comprise the step of (e) reading out charges corresponding to a dark latent image from the image recording medium before performing the step (c). The dark latent image means a latent image caused by unnecessary charges which does not carry image information to be recorded. Generally, the dark latent image includes charges left by a previous reading operation, and a component caused by a current generated in response to application of an electric field to the photoconductor.
Further, it is further preferable that the step (e) is performed after the step (b).
(2) According to the second aspect of the present invention, there is provided an apparatus for recording image information carried by an electromagnetic wave as a latent image in an image recording medium including a photoconductor which is made of an amorphous material, and generates latent-image charges when the photoconductor is exposed to the electromagnetic wave during application of a first electric field to the photoconductor, and a charge storing portion which stores the latent-image charges so as to form the latent image; the apparatus comprising: an electric-field applying unit which applies an electric field to the photoconductor; and a control unit which controls the electric-field applying unit so as to first apply a second electric field to the photoconductor for a certain duration, and thereafter apply the first electric field to the photoconductor for recording the image information, where the second electric field is stronger than the first electric field.
Preferably, the apparatus according to the second aspect of the present invention also has one or any possible combination of the following additional features (iv) and (v).
(iv) The control unit may control the electric-field applying unit so as to apply a third electric field to the photoconductor before application of the second electric field to the photoconductor, where the third electric field has an identical strength to the first electric field.
(v) The apparatus according to the second aspect of the present invention may further comprise a reading unit which reads the image information by detecting charges corresponding to the latent-image charges and being stored in the charge storing portion, and the control unit may control the electric-field applying unit and the reading unit so that the reading unit reads out charges corresponding to a dark latent image from the image recording medium, before recording the image information.
(3) According to the third aspect of the present invention, there is provided a method comprising the steps of: (a) applying a first electric field to a photoconductor made of an amorphous material; (b) stopping application of the first electric field to the photoconductor; and (c) reading image information carried by an electromagnetic wave by exposing the photoconductor to the electromagnetic wave while applying a second electric field to the photoconductor, and detecting charges which are generated in the photoconductor when the photoconductor is exposed to the electromagnetic wave during application of the second electric field to the photoconductor, where the first electric field is stronger than the second electric field.
When the application of the first electric is stopped in the step (b), the electric field applied to the photoconductor is reduced to the strength of the second electric field or less. For example, the electric field applied to the photoconductor may be reduced to the strength of the second electric field, or to zero.
Preferably, the method according to the third aspect of the present invention also has one or any possible combination of the following additional features (vi) to (viii) .
(vi) The step (c) may be performed within about thirty seconds of the operation of the step (b).
(vii) The method according to the third aspect of the present invention may further comprise the step of (d) applying a third electric field to the photoconductor before performing the step (a), where the third electric field has an identical strength to the second electric field. In this case, it is further preferable that the third electric field is applied after a dark current which flows in response to the application of the third electric field becomes stable (i.e., after the photoconductor enters a high resistance state).
(viii) The method according to the third aspect of the present invention may further comprise the step of (e) reading out a portion of a dark current increased by application of the first (preliminary) electric field, before performing the step (c).
(4) According to the fourth aspect of the present invention, there is provided an apparatus comprising: a photoconductor which is made of an amorphous material, and generates charges when exposed to an electromagnetic wave during application of a first electric field to the photoconductor; an electric-field applying unit which applies an electric field to the photoconductor; a reading unit which reads image information carried by the electromagnetic wave by detecting the charges generated by the photoconductor during the application of the first electric field to the photoconductor; and a control unit which controls the electric-field applying unit so as to first apply a second electric field to the photoconductor for a certain duration, and thereafter apply the first electric field to the photoconductor for reading the image information, where the second electric field is stronger than the first electric field.
Preferably, the apparatus according to the fourth aspect of the present invention also has one or any possible combination of the following additional features (ix) to (xi).
(ix) The control unit may control the control unit and the reading unit so that the reading unit reads the image information within about thirty seconds of completion of the application of the second electric field to the photoconductor.
(x) The control unit may control the control unit so as to apply a third electric field to the photoconductor before application of the second electric field to the photoconductor, where the third electric field has an identical strength to the first electric field.
(xi) The control unit may control the control unit and the reading unit so that the reading unit reads out a portion of a dark current increased by application of the second electric field, before detecting the charges generated by the photoconductor during the application of the first electric field to the photoconductor.
(5) The advantages of the present invention are explained below.
(a) According to the present invention, the electric field for recording or reading (i.e., the above first electric field) is applied to the photoconductor after a preliminary electric field (i.e., the above second electric field), which is stronger than the electric field for recording or reading, is applied to the photoconductor. As a result, the dark current level is lower than the dark current level achieved without the application of the preliminary electric field, for a certain duration after the termination of the application of the preliminary electric field. That is, a low-dark-current state is transiently realized. Therefore, when image information is recorded or read while the dark current level is low, it is possible to reduce the dark current component without damaging the true signal component carrying true image information. That is, an image can be recorded or read with a high S/N ratio.
(b) As described before, the low-dark-current state is realized only transiently, i.e., the low-dark-current state is not maintained for a long time. Therefore, it is preferable that the recording or reading of the image information is performed within about thirty seconds of the termination of the application of the preliminary electric field. Usually, it takes approximately one to ten minutes for the dark current to reach a stable leakage current level after the application of the preliminary electric field is completely stopped. Therefore, completion of the recording or reading of the image information within about thirty seconds of the termination of the application of the preliminary electric field makes sure that the recording or reading of the image information is performed in the low-dark-current state. In addition, since the dark current level is very low within about thirty seconds of the termination of the application of the preliminary electric field, the S/N ratio of the recorded or read image is further increased.
(c) As explained before, when an electric field for recording or reading is applied to the photoconductor, the dark current gradually decreases toward a stable leakage current level. However, when a preliminary voltage, which is stronger than the electric field for recording or reading, is momentarily applied to the photoconductor before the dark current reaches the stable leakage current, current injection from electrodes and trapping of the injected charges as space charges (accumulation of space charges) are accelerated by the strong electric field. That is, a state of space charge accumulation which is close to the state of space charge accumulation achieved by long-time application of the electric field for recording or reading can be realized in a short time. In other words, it is possible to reduce the time required for realizing the stable, high resistance state. Thus, it is preferable that an electric field having an identical strength to the electric field for recording or reading is applied to the photoconductor before application of the preliminary electric field. It is further preferable that the preliminary electric field is applied after a dark current which flows in response to the application of the above third electric field becomes stable (i.e., after the photoconductor enters a high resistance state). In this case, the duration in which the dark current level is low becomes long, and the effect of the present invention can be achieved even when the difference between the preliminary voltage and the reading voltage is small. Therefore, from the viewpoint of stability, it is preferable that the preliminary electric field is applied after the dark current which flows in response to the application of the above third electric field becomes stable.
(d) Since the charges stored in the charge storing portion before recording the image information constitutes a dark latent image component, the dark latent image component can be reduced when the charges corresponding to a dark latent image is read out from the image recording medium before recording the image information. In particular, at the beginning of the application of the second (preliminary) electric field, the dark current momentarily increases, and therefore the increased portion of the dark current is likely to be accumulated as a dark latent image in image recording systems. However, when the charges corresponding to a dark latent image is read out from the image recording medium, the influence of the increase in the dark current due to the application of the preliminary electric field can be eliminated. Further, as additional effects, an afterimage formed with charges left by a previous reading operation and photovoltaic noise can be suppressed.
It is further preferable that the operation of reading out charges corresponding to the dark latent image is performed after the termination of the application of the preliminary electric field.
(e) Although charges corresponding to a dark latent image are not accumulated in a sensor (detector) element including the photoconductor in the image reading systems, the dark current increases in response to the application of the preliminary electric field. In particular, the dark current momentarily increases at the beginning of the application of the preliminary electric field. A portion of the dark current increased by the application of the preliminary electric field may be output, and become a noise source. However, a portion of the dark current increased by the application of the preliminary electric field is read out and discarded before reading the image information, the influence of the dark current component increased by the application of the preliminary electric field can be eliminated.